Air conditioning apparatus



June 7, 1960 A. H. ROBSON 2,939,295

AIR CONDITIONING APPARATUS Filed Dec. 29, 1958 2 Sheets-Sheet l FlG.l

1N VEN TOR.

AUBREY H. ROBSON awed ATTORNEY A. H. ROBSON AIR CONDITIONING APPARATUS June 7, 1960 Filed D90. 29, 1958 sAiR FLOW??? 2 Sheets-Sheet 2 INVENTOR.

AUBREY H. ROBSON ATTORNEY United States Patent AIR CONDITIONING APPARATUS Aubrey H. Robson, Rock Island, 111., assignor to American Air Filter Company, Inc., Louisville, Ky., a corporation of Delaware Filed Dec. 29, 1958, Ser. No. 783,478

9 Claims. (Cl. 62'-140) This invention relates to air conditioning apparatus of the type particularly adapted to provide cooled air for use in ground support of aircraft and the like. Specifically, the invention relates to an air conditioner having several air cooling coils or evaporators arranged in series or tandem to obtain relatively low air temperatures, and having means for automatically controlling the direction of air flow and supply of refrigerant to defrost the coil upon which frost accumulates.

It is known to utilize two or more coils in a tandem arrangement wherein the air flows in one direction until the downstream coil accumulates an objectionable load of frost, and then to reverse the air flow so that the previously downstream coil is defrosted by the flow of unconditioned or relatively warm air.

It has been suggested that the need for a reversal of air flow can be sensed by using a temperature sensitive element in the discharge air stream. One such 'arrangement apparently is based upon a theory that an increase in air outlet temperature indicates a decreased effectiveness of heat transfer of the frosted coil and a consequent need of defrosting. Another such arrangement apparently is based upon a theory that a decrease in air outlet temperature indicates a decreased air flow giving more heat transfer per unit volume of air, the decreased air flow resulting from the increased resistance to air flow exerted by a frosted coil and consequently signalling for a need to defrost. While no judgment is made as to the correctness of either of these seemingly contradictory concepts, it is at least apparent that substantial difiiculties would be encountered in the adaptation of such control to an air conditioner required to provide air at selected low temperatures within a range of temperatures.

It has also been taught that the supply of refrigerant to the frosted coil should be cut off'to increase the speed of defrosting. While rapid defrosting can thus be accomplished, it is obvious that one spare-coil must be provided in addition to the number of coils necessary to give the desired cooling capacity. This spare coil adds expense and poses space requirement problems; particularly with air conditioners of the portable type.

Therefore, one object of the present invention is the provision of an air conditioner having a plurality of cooling coils arranged in series and wherein the direction of air flow is controlled in response to the difierence in air flow resistance or static air pressure drop across each of the coils.

. Another. object is the provision of an air conditioner wherein refrigerant will normally continue to be supplied to the frosted coil after reversal of air flow.

Still another object is the provision of an air conditioner having means for detecting a failure of the frosted coil to defrost with sufficient rapidity, and having means to control the air conditioner in a manner to increase the rateof defrosting in response to'such a condition;

- Still another object of the present invention is the integration of the foregoing into a temperature control 2,939,295 Fatented June 7, 1960 ice.

system of the type which permits the selection of a desired outlet temperature within a range of temperatures.

In accordance with the present invention, there is provided switch means responsive to the difierence in air pressure drop across, or resistance to air flow exerted by, each of the coils arranged in series, the switch being operable in response to a predetermined differential between the pressure drop of the two coils to actuate means for operating suitable damper means to reverse the direction of air flow through the coils. The damper operating means is arranged so that the movement of the dampers to an opposite extreme position continues after damper reversal starts irrespective of the changing pres-- sure differential during the damper movement, and irrespective of the reversed pressure differential dampers have been reversed.

In accordance with another feature of the invention, the pressure responsive switch means remains, for a' sufiiciently long period, in a position opposite to that position which causes reversal of the damper means, thereby indicating that the frosted coil is not being defrosted at a sufficiently rapid rate, that this condition be utilized to reduce temporarily the refrigerant supply so that the frosted coil will more quickly defrost. Further in accordance with the invention, the refrigerant system includes separate suction lines connected to an isolated bank compressor so that thermostatic expansion valves in the refrigerant supply lines can function -'to distribute the refrigerant supply to the coils in dependence upon the heat load imposed upon the individual coils.

The invention is described in connection with the accompanying drawing which illustrates one embodiment of. the invention by way of example, and wherein:

Figure 1 is a diagrammatic view of an air conditioner embodying the present invention;

Figure 2. is a view of asuitable pressure responsive switch with the cover thereof removed;

Figure 3 is an electrical circuit diagram of a control system for the air conditioner.

Referring to Figure 1, a suitable air blower 2 supplies unconditioned or ambient air through inlet duct 4 to one side of cooling tank 6 which is divided by a partition 8 into chambers 10 and 12 and which has an intermediate chamber designated 14. Cooling or evaporator coils 16 and 18 are mounted to extend horizontally and transversely across chamber 10 and 12 respectively." Each of the chambers 10 and 12 is connected by ducts 20 and 22 respectively, to a common air outlet duct 24.

At the inlet and outlets of cooling tank 6, a set of dampers including air inlet damper 26 and cooling tank air, outlet dampers 28and 30 are suitably linked as indicated by the dotted line to be operated simultaneously to close off the outlet of one chamber and the inlet of the other. With the dampers positioned as shown by solid lines in Figure 1, the incoming air is directed by damper 26 into chamber 10, through coil 16, chamber 14, coil 18, chamber 12, and thence through duct 22 to the outlet 24. It will be apparent that if the dampers are displaced to their opposite extreme position (dotted line positions), the air will flow into the tank byway of chamber 12 and leave the tank by way of duct 20 from chamber 10. The position of the dampers is controlled by damper operator 32. An outlet temperature sensing element 34 disposed in the outlet 24 is a part of a temperature control system to be hereinafter described.

The refrigeration system includes an isolated bank compressor 36, a condenser 38, a receiver 40 from which liquid refrigerant is supplied through line '42 to separate when the Refrigerant flow to coils 16 and 18 is controlled by conventional thermostatic expansion valves 52and 54 in the separate coil supply lines 44 and 46 respectively. These valves are responsive to the refrigerant gas temperaturein the" suction lines 48 and 50. To this end, a temperature responsive element 56 is disposed adjacent suction line 48 and is connected by capillary tube 58 to valve 52, and temperature responsive element 60 is likewisedisposed adjacent suction line 50 and is connected to walves t by capillary tube 62.

wAsris wellknown in the art, such expansion valves respond to degrees of superheat of "refrigerant gas leaving the-coils and tend to open in response 'to' 'a' starved condition in. the coils and tend to close in response to a fioodedzcondition. The conditionof the refrigerant gas leaving the coils is a function of the heat load imposed thereon. That is, with an increased heat load imposed onY-a coil the gas-has an increased degree of superheat; and conversely, with a decreased heat load i'rnposedon a ,coilthe gas has a decreased degree of superheat. Thus,

that the static pressure in each pressure switch chamber will be the same as in the cooling tank chamber connected therewith.

Referring to both Figures 1 .and 2, and assuming air flow through the cooling tank in the direction shown by the arrows in Figure 1, it will be. apparent that the static pressure in chamber '10 will exceed the static pressure in chamberl'l l because of the resistance to air flow exerted by coil 16, and the static pressure in chamber 14 will exceed the 'staticpressure in chamber 12, due'to the resistance to the same air flow exerted by coil 18. Assuming that neither coil is frosted, the pressure drop across coils 16 and 18 will besubstantially the same and the, valves'52 and 54 a re'efiectively controlled by the imposed heat load on each coil. f

faculty :of. thermostatic expansion 'valvesis emplo/yedto advantage inaccordan'ce with the inventionby utilizing separate suction lines connected to separate cylinders or. banks of cylinders of the compressor 36 so that the pressures and consequently superheat, in the gas leaving the coils, can differ from one suction line tothe next.v n V t V .Assurning'the required. final outlet air temperature is 7 not below a certain limit, the heat load imposed upon the upstream coil is greater than that imposed upon the downstream coil- Therefore, the refrigerant in the upstream'. coil boils or evaporates at a faster rate than .the refrigerant in the downstream coil,- and the upstream coil temperature will be higher than the downstream coil temperature.

Since with separate suction lines the compressor dis placement offered to each coil is identical, the compressor operation pulls down or increasesthe suction. pressure in the downstream coil to'a pressure below that pressure possible if a common suction line were used. Hence the downstream coil temperature -is depressed to a'value below that attainable with a common suction line. In

this way it ispossible to operate with a downstream coil temperature well belowfreezing and an. upstream coil temperature above freezing, Thus air can be cooled to a temperature below freezing while the rate of frosting on the downstreamcoil is suppressed and frosting on the upstream coil is prevented. i i

Referring further to Figure 1, a pressure responsive switch designated 64 has four separate interior chambers in communication though pressure lines with selected portions of the-cooling tank 6. Pressure line 66 connects the switch with the chamber '10 of the tank, lines 68 and 70 connect intermediate chamber 14 of the tank to the switch, and line 72 connectsthe switch to chamber 12 "The pressure switch 64 is shown in Figure 2 with its iront'cover removed to show internal construction. The rotatably mounted vane piston 74' divides the interior of the switch into the four interior ichambe'rsjsaled from e'achrother and designated as 76, 78; 80 and SZ. The time 74 is suitably biased to" 'a'nfintenne'difat position. Each of the pressureswitchcha'mbers is fin chrniiiunic'ation with a selected'par't'ofa c oling tank by m'eafis or the aforementionedpressure-dines wa es-'70 and 12, 59

mediate position as shownin Figure 2. As the downstream coil 18 accumulates frost, its resistance to air flow rises and the pressure drop thereacross consequently increases; The upstream coil 16, which is operating with its'co'oling surfaces above freezinggdoes not frost up andtherefore has less resistance to air flow-and a lower pressure drop thereacnoss than coil 18.

The operation of the pressure switch 64 in response to such variations in cooling tank chambers 12, 14 and 16 will now be described; Since the static pressure in switch chamber 76 exceeds the static pressure in switch chamber 78, this difierentialtends to 'move the vane piston in a counterclockwise direct-ion. However, the static pressure in switch chamber 89 exceeds the static pressure in switch chamber 82 by an even greater amount and this greater differential tendsto rotate the vane piston in a clockwise direction. 'When the differential is great enough to overcome the means biasing the vane piston to a 'neutral'position, the vane piston will rotate in a clockwise direction to a degree corresponding to this difierential. As the coil 18 continues to frost, thisforce will correspondingly increase until a predetermined frosted condition, the vane piston will be rotated to its extreme clockwise position. The displacement of the vane vpiston to its 'extreme'clockwise position is used, in accordance with the present'inventio'n, to indicate that the downstream co'il.18 should be defrosted to prevent further obstruction to air flow, the defrosting being accomplished by reversing the damper positions to reverse the direction of air flowthough the cooling tank.

The operation of the. dampers for reversing the 'air flow in response to predetermined displacement of the vane piston-is accomplished through thecircuit shown in Figure 3. A vane switch designated 84, and connected to electrical'power soiifce 86, is linked for operation with vane piston 74 so that as 'vane piston rotates in either direction, the vane switch 84 correspondingly rotates. The vane switch 84 may suitably be a mercury switch mounted on the axis of thetvane piston 74'fo'r a tilting rotation in onedirection or another with rotation of the vane piston. Inaddition to being connected to the common moving terminal of vane switch 84, power source 86 is also directly connected to intermediate contact segments of motor reset switches '88 and 90 which form part of the circuit for controlling operation of the servomotor damper operator32; This circuit also includes switches 92 and 94 which are controlled by the displacement of the 'damper, operator 32, limit switch 94 opening when the 'damper operator is operated to its extreme position corresponding to the position 'of dampers in Figure l, and limit switch 92 opening at the epposite extreme position; his to be understood that the'wipin'g contacts or arms hf reset switches 88 and 90 are also rotata'bly displaced as 'servomotor 32 is operated.

For purposes of clarity in explanation, the angular disposition of these wiping contacts will generally correspond with the angu1ar=dispositionof the dampers 26, 28

l, and 30.

When th piston 74 "assumes its. clockwise position due to the aforementioned static pressures beingjinipos'ed on the vane pistonfaces, vane switch 84 assumes a "con responding clockwise position and power is delivered to terminals 98 and 100 of the vane switch. The power delivered to terminal 98 energizes the servomotor 32 through the left end segment of motor reset switch'88, the limit switch 92, and causes the servomotor to operate in a direction to displace the dampers from the solid line position shown in *Figure 1, to the dotted line extreme opposite position. As the servomotor displaces the dampers in this direction the static pressure conditions initially causing the energization of the servomotor are disturbed. Thus, the vane piston 74 moves from its extreme clockwise position and consequently the vane switch 84 breaks contact with terminals 98 and 100. However, the servomotor remains energized through the middle contact segment of reset switch 88 and continues the displacement of the dampers until they are displaced to their opposite extreme position whereupon limit switch 92 and motor reset switch 88 open to deenergize the servomotor, and limit switch 94 closes to permit subsequent energization of the servomotor for operation in the opposite direction.

' Since the frosted coil 18 is now the upstream coil and has a resistance exceeding the resistance across coil 16, the vane piston 74 and vane switch 84 will have .responded to the reversed pressure conditions by assuming extreme counterclockwise positions. The unconditioned or relatively. warmer air now passes through the up stream coil 18 first and serves to defrost it. As the coil 18 defrosts, the vane piston 74 and vane switch 84 will correspondingly move towards an intermediate position from the extreme counterclockwise position. When the upstream coil 18 has completely defrosted and the now downstream coil 16 accumulates sufiicient frost that the pressure drop across the downstream coil exceeds the pressure drop across the upstream coil by a value sufficient to cause the vane piston 74 to move again to an extreme clockwise position, power to operate the servo motor 32 will be delivered through switch terminal 100 connected to rig-ht end contact segment of motor reset switch 90, and the closed limit switch 94.' Thus, the

dampers will be displaced to their original positions as shown in Figure 1 and frosted coil 16 again becomes the upstream coil and coil 18 the downstream coil. This manner of reversing air flow will continue so long as the coil which is downstream with respect to air flow continues to accumulate suflicient frost to displace vane piston 74 to its clockwise position.

. While the foregoing arrangement normally permits both coils 16 and 18 to remain in service continuously to provide cooling air, certain conditions may be encountered which prevent the upstream coil from defrosting as rapidly as is desired. Such a condition may arise, for example, when the ambient air being conditioned is relatively humid and of a low temperature. In such a case, it would be possible that the upstream coil would retain a substantial quantity of frost while the downstreamcoil also accumulated frost. This would hinder proper control and also impose an additional resistance to air flow. Thus, under certain conditions it may be desirable to defrost the upstream coil more rapidly than is possible by simply utilizing the unconditioned air passing the coil. This is accomplished by reducing the available refrigerant supply to the coils until the frosted conditionv of the upstream coil is relieved. One preferred way of doing this is by reducing compressor speed.

While control of outlet air temperature is temporarily sacrificed by unbalancing the temperature control system, the temperature control is aflfected for a short time only.

Referring to Figure 3, there is shown a conventional Wheatstone bridge which includes the air flow outlet temperature sensing resistor 34 in one leg, an air flow temperature selecting resistor 102 in another leg, and several balancing resistors in the opposite legs. The output terminals 104 of the Wheatstone bridge are connected to a polarized relay 106 which controls a single pole, double throw switch 108 suitably biased to an intermediate position. The switch 108 controls the energization of the servomotor 110 which in turn controls the position of a variable resistor 112 in series with a field coil 114 of a magnetic particle clutch 116. The clutch 116 controls the degree of coupling between the driven compressor 36 and the driving prime mover 118.

The operation of the Wheatstone bridge in controlling the speed of the compressor through the magnetic clutch will now be described. If the outlet temperature sensed by resistor 34 exceeds the selected temperature established by resistor 102 by at least a predetermined value, the imbalance in the Wheatstone bridge will cause current to flow through polarized relay 106 in a direction to close the switch 108 tothe terminal which energizes servomotor 110 for operation in a direction to decrease the resistance of resistor 112. Thus, the current flow through coil 114 increases and the degree of coupling eifected by clutch 116 consequently increases, thereby increasing the speed of the compressor 36. The increase in the speed of the compressor 36 will provide the additional refrigerating capacity to reduce the outlet temperature to that established by resistor 102.

Conversely, if the temperature sensed by resistor 34 is sufficiently below that established by resistor 102, the current fiow through the polarized relay 106 will be in the opposite direction, the switch 108 will close to the opposite terminal and servomotor 110 will be operated to increase the resistance of resistor 112. The decrease in current through field coil 114 decreases the coupling of clutch 116 so that the compressor speed is reduced and the refrigerating capacity of the coils is decreased.

This temperature control system-is utilized in connection with the air flow reversing system to provide rapid defrosting when required. To this end, the vane switch 84 is arranged so that it is in its extreme counterclockwise position (corresponding to a reversal of air flow dampers so that the frosted coil is now upstream), the switch 84 will close to terminal 120 to provide current flow to a resistor 122, a heating element 124' as-' sociated with the resistor 122, and a bias relay 126 disposed, along with polarized relay 106, to exert control over switch 108.

The resistor 122 is chosen so that its resistance in an unheated condition is relatively high and current flow to relay -126 is negligible. Thus, switch 108 is normally How- 1 ever, as resistor 122 is heated by its associatedheating element 124, its resistance decreases and current flow therethrough correspondingly increases.- Thus, when vane switch 84 is displaced to its extreme counterclockwise position the heating. element 124 is energized and begins to heat the resistor 122. If the vane switch 84 remains in its counterclockwise position for at least a predetermined period (thus indicating that for some reason the frosted upstream coil is not being defrosted rapidly enough), the resistance of resistor 122 will have decreased enough that the current flow to bias relay 126 actuates switch 108 to a position causing a reduction in compressor speed through the servomotor 110, its controlled resistor 112 and the magnetic particle clutch coil 114. f While control of'outlet' temperature by the .Wheatstone bridge'is disturbed when bias relay' 126 actuates switch 108, the defrosting of the upstream coil is accomplished more rapidly and loss of temperature control is for a short time only. The control is of course restored to the Wheatstone bridge as soon as the upstream coil defrosts to a degree causing switch 84 to move from its extreme counterclockwise position and break contact with terminal 120.

The embodiment of the invention herein described by wa' 'sr example" is particularly suited for use as the air cooling part of an all weather-air conditioner-adapted to. provide either heated or cooled air, 'or both. Such all weather air conditioners may be required to operate in diverse geographical areas having ambientternperatu'resas low as -65 F. and as high as 120 F. Further, the-air conditioners may be required to provide high pressure, low temperature air to a missile or aircraft ventilating system. Because of .the'need at times for cooling air at a-high pressure, the'cooling system may be required to operate when the ambient air is below freezing since the .pre'ssurizin'g of air increases its temperature considerably. Thus, it will bec'lcar how the 'air conditioner can be required to provide cooling air when the ambient tem eratures below freezing. It

vis only necessary that the temperature 'of'the pressurized air passing through the'upstream coil is enough to melt accumulated frost. c v, v. s

Having described my iriventioniI'clainm a 4 [11 'In an air conditioner; a plurality" of a'i'r'co'olin'g 'c'oils' arranged to receive air flow in series.,'seqi1ence;a're- 'frigerant system including means for supplying refrige rant to said coils; damper means dis'pilaceable to alternate positions for reversing thefsequefice in which air 7 flows through said 'c'oirs; means for detecting an imbalancein'resistance to 'air flow exerted by each of said coils occupying the downstream and upstream position;

7 and meansresponsive to an imbalance in one direction corresponding to the air flow resistance exerted by said downstream coil exceeding the "air flow resistance exa refrigerant system includes, an isolated 'bank compressor havingeach of said isolated banks connectedby separate refrigerant suction lines to said coils, and thermostatic expansion valves in the supply line of each of said coils for distributing the supply of available refrigerant in a'cco'rdance'with the heat load imposed on. each of said coils,

air cooling coiIs includingj'a'n upstream and downstream coil; a refrigerating system for supplying liguified refrigerant to said coils and receiving said gasified refrigerant'therefrom; damper means operable to reverse the direction of air flow through said coils so that the sequence in which said coils receive said air flow is reversed; air pressure responsive switch means displaceable in a first direction in response to a static air pressure drop across said coil occupying the downstream position exceeding a static air pressure drop across said coil t occupying the upstream position, and in the opposite direction in' response to a reversed differential in static pressure drop across said coil; and, means'for operating said damper means to reverse the direction of air flow in response totdisplacement of said .air p'ressureswitch means a'predetermined degree in said first direction cor- 4. in anair conditioner: 'aplurality of series-arranged responding'to a predetermined frosted condition of said downstream coil a e V a a 5. Anair conditioner as specified in claim 4ineluding; means for reducing said refrigerant supply to said-coil in response to displacement of said air pressure responsive SWliCl'lD'lEfl-HS a predetermined degree in said opposite direction 'for a predetermined sustained period indicating failure of. said coil occupying the'upstream position "to defrost sufiiciently rapidly. V it 6. In an air conditioner: a housing defining an air flow passageway; a .plurality.of series-arranged air cooling coils inrrsaidhousing; damper means for reversing the direction o'f-air flow through said housing; pressure responsive means placingthe static air pressure drop across the stream coil;

downstream coil and across the upstream-coilwof said series-arranged coils in opposition; and means for. operating said damper means 'to .a reversed position inrcspouse to a condition -'ofsaid pressure responsive means corresponding to said static air :pressure drop across said downstream coil exceeding said static air pressure drop across said upstream coil by. -apredetermined value indicating a corresponding degree o'f-frosting of said down- 7. conditioner comprising: blower means prm viding'a flow of air to betempered; a housing defining a passageway for the air flow from said blower means; a plurality of series-arranged air coolingcoils i-n-said housing; a refrigerant system for supplyingsrefriger'ant t c-said coils; air temperature controlsmeans operable to vary the supply ofrefriger ant to said coilsin response tdVaria'tions in thesoutlet temperature ofsaid air flow; damper means operable to reverse the direction of air flow throughfsaid housing; meansresponsive to a predetermined diflerential in static air pressure drop across the upstream and across the do'wnstreamcoil respectively correspondingto 'a predetermined frosted condition of saiddownstream 'coil'to operate said damper means to reverse the direction of air flow; and means'for overriding said temperature control means to-reduce 'thessupply of refrigerant to saidcoils in response. to. a sustained reversal of said predetermined diiferential insaid static air pressure drop corresponding to a sustained frosted condition ofsaid upstream coil.

8.7 Thejair conditioner of claim, 7 wherein: said means responsive .to the difieren'tialsinr static air pressure drop across said coils includes switch means operable, in one position, to reverse s'aid'dan'ipermeans, and in anopposit'e position, toenergize said overriding means. a y 9. The air conditioner'of clainrS wherein: said air temperature controlmeans-includes-a bridge, eircuit operativetoproduce-an output signal in accordancewith adeparture in outlet air temperature; from a selected air te'mperature said output 'signalbeing operative to exert con trol fov'er refrigerant compressor speed switch means-and, said overriding means includes means operative to ov'erridetcon't'rol ofsaid refrigerant compressor speed switch 7 means by said bridge circuit output signal in response to a predetermined sustained displacement of said'air pressure switch 'means inv said opposite position.

References Cited in the file of'this patents UNITED STATES PATENTS 

